人体有3种血管：动脉、______和______．手臂上的一道道“青筋”是______血管．毛细血管是血液和细胞间物质交换的场所，连通于最小的动脉和最小的静脉之间，管壁只由______层上皮细胞构成．
血管分为动脉、静脉和毛细血管三种，动脉是将血液从心脏输送到身体个部分去的血管，静脉是把血液从身体各部分送回心脏的血管，有的静脉与动脉伴行，位置较深，有的静脉位置较浅，在体表可以看到，如手臂上的一条条青筋，就是静脉．毛细血管是连通于最小的动脉与静脉之间的血管，只允许红细胞单行通过，毛细血管的管壁只由一层上皮细胞构成，管内血液流动最慢．这样的结构特点便于血液与组织细胞之间进行物质交换．
故答案为：静脉；毛细血管；静脉；一．
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Ric kshaw pullers (人力车夫) are some of the poorest people in our city and they work extremely hard. Recently I suffered a(n)__1__injury and had to use a rickshaw puller to take me to the hospital every __2__.I also had to use these trips to get all the stuff (物品) I__3__like medicines, milk, fruit, etc.__4__ I couldn't walk too much.Every day on the way back home, I would__5__the rickshaw puller to stop by the__6__to get me the things I needed. Each day, it would be a__7__rickshaw puller and at first I was worried that they would not__8__to stop or they might ask me for more money for this.However, they__9__me wholeheartedly; without any __10__of the time, effort or money, the rickshaw pullers would__11__set off to get the stuff for me.Often they weren't exactly__12__about what to get so they would come back to me for clarification (说明). Sometimes they would__13__different kinds of stuff with them to show me so I could select__14__and then they would go back to the shop to buy it.They would take care to make sure I got whatever I needed at the right price.They would try to__15__the rickshaws as close to my __16__as possible, so I would have to walk as little as possible to get into my house.One of them didn't even take any extra (额外的) __17__for the time or efforts to help me when I__18__it at the end of the trip. I was so__19__these rickshaw pullers for offering me not only help but care and concern in a genuine (真诚的) way, as if I were their__20__. | ||||
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